Merge pull request #567 from cedar-renjun/patch-5

[BSP] Update and rename readme.txt to readme.md in stm32f4

Author: BernardXiong

bsp/stm32f40x/readme.md

@@ -0,0 +1,25 @@
+board info:
+STM32F4 discovery
+http://www.st.com/internet/evalboard/product/252419.jsp
+
+note:
+in drivers/stm32f4xx_conf.h
+ /* Redefine the HSE value; it's equal to 8 MHz on the STM32F4-DISCOVERY Kit */
+ #define HSE_VALUE    ((uint32_t)8000000)
+ 
+ 
+ 
+ **================= IAR WARNING =================**
+ 
+ after use `scons --target=iar -s` cmd to product IAR project, you have to add iar link file by hand
+ 
+ IAR steps:
+ 1. Project-->Options-->Linker
+ 2. Config-->Linker Configuration file
+ 3. select `Override Default`, then select `bsp/stm32f40x/stm32f40x_flash.icf`
+ 4. rebuild project
+ 
+ 
+ **TODO**
+ auto add *.icf by scons script
+